Heat Index Calculator

Calculate apparent heat index temperature from air temperature and relative humidity using the official NWS Rothfusz regression equation.

How to use this calculator

1. Enter air temp ≥ 80°F.
2. Enter relative humidity %.
3. Read feels-like heat index and warning category.

Formula & method

NWS Rothfusz Regression

HI = Multi-variable polynomial regression of Temp & RH

NWS heat index model.

Example

90°F at 65% humidity = 102.9°F Heat Index.

Key insights

• High humidity prevents sweat evaporation.

How to interpret your result

Heat Danger

NWS heat stroke alert category.

Frequently asked questions

Min temperature?

Valid for temps ≥ 80°F (26.7°C).

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